Populus "androscoggin" : some wood characteristics and drying options

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Thesis Discipline

Forestry

Degree Grantor

University of Canterbury

Degree Level

Masters

Degree Name

Master of Forestry Science

This study explored the wood properties of Populus "androscoggin" (Populus trichocarpa X Populus maximowiczii) grown in a river bed plantation near Timaru.
The objectives of this study were:
(1) to determine some general characteristics of the tree viz. diameter over bark, diameter under bark, bark thickness, eccentricity and the diameter of heartwood.
(2) to improve the quality of sawn timber by drying the timber so as to reduce the effect of tension wood and growth stresses in poplar.
Thirty trees were selected for this study 15 leaning trees and 15 straight trees. The study was conducted in three parts:
(1) disk analysis to determine the general wood characteristics
(2) determining drying characteristics based on the quality of dried flitches
(3) determining the quality of dried boards after remanufacture.
In the first part of the study sample disks were removed at three heights from each tree to examine the variation within the tree. The mean number of annual rings at the butt of the tree was 16 in the case of leaning trees and 17 in straight trees. The growth rate (ring width (mm)) of the leaning trees was not statistically different from the growth rate of straight thickness. It was found that thickness was positively correlated with the diameter over bark. Bark thickness also has a positive relationship with heartwood diameter and negative relationship with height.
The average green moisture content of individual trees ranged from 109 % to 146 %. The moisture content decreased from the butt green to the top of the tree. A positive relationship was found between the green moisture content and the proportion of heartwood (heartwood diameter/the diameter under bark).
The green density followed a similar trend to that of the distribution of green moisture content, and decreased from the butt to the top of the tree.
Basic density values in the 30 trees were,
- butt: 320 to 380 Kg/m³
- middle: 340 to 410 Kg/m³
- top: 360 to 430 Kg/m³.
It was found that the distribution of basic density based on the "direction" in the tree was different in straight trees and leaning trees. In leaning trees the basic density of the "side" was statistically different from that of the "opposite" direction, whereas in straight trees there were no differences between directions. It was also found that the basic density of leaning trees was higher than the basic density of straight trees.
Within the stem, basic density was negatively correlated with the heartwood proportion, negatively correlated with the average ring width, and positively correlated with its eccentricity.
In the second part of the study, the logs from the butt and the top were converted into flitches. The flitches were then dried using six drying schedules. One of the drying options (dehumidifying) was not completed due to time limitations.
Air drying schedule which was begun on 24 April 1989 finished on 30 March 1990 (11 months), with the average moisture content ranging from 17.2 % to 25.8 %. The other four categories of drying options were low temperature drying (40°C), conventional drying I (60°C), conventional drying II ( 80°C) and high temperature drying (115°C). When dried at low temperature the boards showed minor defects (warping), conventional drying I and conventional drying II decreased the drying time but the effect on defects was not significant. High temperature drying decreased the drying time and also decreased the amount on warp of the dried flitches.
More boards were rejected from leaning trees than from straight trees. More boards were rejected from bottom logs than from top logs.
Volumetric shrinkage was not affected by the type of tree nor the location of the board within the tree. On the other hand drying methods (temperature) had an effect on volumetric shrinkage.
Below 100°C, it was noted positively that the volumetric shrinkage was correlated with temperature, whereas at high temperatures (> 100°C) the opposite was true. This result suggested that the allowance before sawing processes should be made in accordance with the drying methods. The higher the temperature (below 100°C) the higher the allowance.
The rejection rate showed a positive relationship with the volumetric shrinkage.
In the third part of the study, 5 samples from each treatment were selected then remanufactured and the distortion measured. The result after remanufacture was different from that obtained in the second part of the study. The high temperature drying showed a high amount of bowing in remanufactured boards. This indicated that casehardening was present in these boards, and suggested that more reconditioning was needed for this schedule (up to 10 hours instead of 6 hours).
Colour was darkened and there was loss of brightness at higher temperatures. The high temperature drying also produce a "caramel” like odour. This made the boards unsuitable for asparagus containers.
Wetwood may be present in the boards used in this study, since the final moisture content varied greatly between and within the boards. Further research of this abnormal wood is needed to clarify this problem.
This study also suggested that the drying methods should consider the requirement of the final product. Timber intended for manufacture of asparagus containers was suitably processed by conventional drying I or conventional drying II, whereas for other end uses (where the odour and colour were not primarily important) high temperature drying was found to be suitable used in conjunction with an increased reconditioning schedule.